In co-ordinate measuring and similar machines an object is measured by relative motion between a probe and an object in a controlled manner, usually from a datum, until a "touch" of the object is indicated by the probe. The co-ordinates of the touch position are recorded. By such repeated actions the dimensions of the object can be determined.
The aim of such measurements is to achieve precision to better than one micron with a repeatability of half a micron or less. Accordingly it is clear that the "touch" action must be very responsive and free from error. Probes to achieve such touch action usually use a light, thin stylus with a ball-end of hard material to touch the object. The touch action deflects the stylus in a mounting in the probe and the occurrence of the deflection of the stylus is sensed to cause the co-ordinates to be recorded.
Known probes provide good results in many measurements but in some measurements errors arise which cannot be compensated by corrections to the recorded co-ordinates. One cause for such errors is that deflections of the stylus in different directions requires different amounts of deflection before sensing occurs. Another cause is that an electromechanical switch operated by the stylus is used for the sensing action and the making or breaking of the switch is not consistent because of arcing or corrosion at the switch contacts. Probes used hitherto have electrical or mechanical connections to the stylus.
UK Patent Number GB 2,220,748A describes a probe with an external constraining means and a sensing arrangement consisting of a mirror on the stylus to deflect light away from a detector on deflection of the stylus.